Phosphor having zinc oxide attached to its surface

ABSTRACT

A phosphor comprising a phosphor and zinc oxide formed in a suspension of the phosphor and attached to the surface of the phosphor.

The present invention relates to phosphors for cathode ray tubes and amethod for treating the surface thereof. More particularly, it relatesto phosphors capable of forming excellent phosphor screens on faceplates, and a method for treating the surface thereof.

As is well known, phosphor screens for color television cathode ray(picture) tubes are composed of blue, green, and red emitting phosphordots or stripes of blue, green, and red emitting phosphor componentsdisposed regularly on a face plate. The phosphor screens for colortelevision picture tubes are prepared by a photo-printing method. Thatis, a first light emitting phosphor component is dispersed in a solutionof a light sensitive resin such as, for example, an aqueous solution ofpolyvinyl alcohol activated with a dichromate, to provide a phosphorslurry. The phosphor slurry obtained is applied over the whole surfaceof a face plate by an appropriate coating method such as by rotarycoating, etc. (slurry coating), and then the coated layer is irradiatedby energy rays such as ultraviolet rays, etc., in conformity with adesired pattern, whereby the resin is hardened and insolubilized at theportions irradiated by the energy rays (light exposure). Thereafter, theresin at the non-irradiated portions (unhardened resin portions) isdissolved away by a solvent, etc. (development) to form dots or stripescomposed of the first light emitting phosphor component. Then, bysuccessively repeating slurry coating, exposure and development in thesame manner as described above using second and third light emittingphosphor components, dots or stripes composed of the second and thirdlight emitting phosphor components are formed on the face plate. In thiscase, as matter of course, the energy ray irradiation must be socontrolled that the dots or stripes composed of each of the first,second, and third light emitting phosphor components are repeatedly andregularly disposed on the face plate without being overlapped with eachother. Then, the fluorescent screen containing the resin component thusprepared is fired at a proper temperature to decompose and volatilizethe resin component, whereby a desired phosphor screen is obtained.

For making phosphor screens for color television picture tubes by aphoto printing method using the above-mentioned phosphor slurries, thefollowing requirements must be met.

1. The phosphor screen must be dense and have few pin holes or the like;

2. One light emitting phosphor component must not intermix with otherlight emitting component phosphors, i.e. color mixing must not occur;

3. The phosphor slurry must have high light exposure sensitivity andmust be easy to work with;

4. Formation of the light emitting elements (dots or stripes) must beexcellent, i.e. the edges must be sharp, the dots must be circular, andthe stripes must be linear;

5. The exposed phosphor screen must not fall off the face plate by highpressure aqueous development treatment during the development process,i.e. it must have a high adhesive strength.

Heretofore, various studies and improvements have been made with respectto the surface treatment of phosphors, the compositions and the methodfor the preparation of phosphor slurries with an aim to satisfy theabove requirements. For example, Japanese Examined Patent PublicationNo. 21675/1985 discloses a phosphor having its surface coated with zinchydroxide for the purpose of improving items 2 and 3 among the aboverequirements.

With respect to the color television picture tubes, there has been anincreasing demand for improving the image quality in recent years. Forthis purpose, it is necessary to form finer picture elements on a faceplate so that a highly fine image can be formed.

To fulfill the above demand, it is firstly required to satisfy therequirement that the adhesive strength of the screen portion hardened bythe exposure is high (above requirement 5) and the requirement thatcolor mixing must not occur (above requirement 2), simultaneously.However, in order to increase the adhesive strength of phosphor screensby conventional methods for forming phosphor screens, a method isusually employed wherein the drying is strengthened at the time ofcoating, whereby there will be a difficulty such that the higher theadhesive strength of the phosphor screen, the more the unnecessaryphosphors tend to remain on other position, thus leading to colormixing. With the above-mentioned phosphor coated with zinc hydroxide, itis hardly possible to satisfy such mutually opposing requirements at thesame time. It is conceivable to increase the amount of zinc hydroxidecoated in order to adequately prevent color mixing in the abovementioned phosphor coated with zinc hydroxide. However, if the amount ofthe coated zinc hydroxide increases, the action for aggregation of zinchydroxide colloid increases in the phosphor slurry, whereby thedispersibility of the phosphor in the slurry deteriorates, which in turntends to lead to a decrease of the adhesive strength, formation of pinholes or a deterioration in the sharpness of the edge.

The present invention has been made in view of the above problems, andit is an object of the present invention to provide a phosphor which iscapable of preventing the occurrence of color mixing of the phosphorscreen and increasing the adhesive strength of the phosphor screen atthe same time and which is excellent in the dispersibility in theslurry, and a method for treating the surface thereof.

The present inventors have conducted extensive researches to accomplishthe above object and as a result, have found that among phosphors havingzinc oxide attached in their surface, those obtained by forming andattaching zinc oxide on the phosphor surface in a suspension of aphosphor, have the above-mentioned desired properties for formingphosphor screens.

Namely, the phosphor of the present invention comprises a phosphor andzinc oxide formed in a suspension of the phosphor and attached to thesurface of the phosphor. Here, "zinc oxide is formed in a suspension"means that zinc oxide is formed by reacting zinc ions and an alkalinesolution in the suspension.

Now, the present invention will be described in detailed with referenceto the preferred embodiments.

In the accompanying drawings:

FIGS. 1(a) and (b) are microscopic enlarged photographs of phosphorparticles having zinc oxide formed in a suspension attached on theirsurface. FIGS. 2(a) and (b) are microscopic enlarged photographs ofphosphor particles having zinc oxide attached merely by mixing phosphorand ready made zinc oxide on their surface.

FIGS. 3(a) and (b) are microscopic enlarged photographs of phosphorparticles having zinc hydroxide attached on their surface.

FIGS. 4(a), (b) and (c) are diagrammatical views of the above-mentionedphosphor particle having zinc oxide attached on its surface, thephosphor particle having zinc oxide deposited on its surface and thephosphor particle having zinc hydroxide attached on its surface.

FIG. 5 is a graph showing the temperature and pH ranges in which zincoxide is formed.

FIGS. 6 and 7 are graphs showing cross-contamination of the phosphorhaving zinc oxide attached on its surface, the phosphor having zincoxide deposited on its surface and the phosphor having zinc hydroxideattached on its surface.

FIGS. 1(a) and (b) show photographs of the phosphor of the presentinvention enlarged 5,000 and 20,000 magnifications, respectively, by anelectron microscope. FIGS. 2(a) and (b) show, for the purpose ofcomparison with the phosphor of the present invention, electroscopicphotographs with 5,000 and 20,000 magnifications, respectively, of aphosphor having zinc oxide deposited on its surface by adding adispersion of commercially available zinc oxide to a suspension of thephosphor, followed by stirring and mixing. Further, FIGS. 3(a) and (b)are electron microscopic photographs with 5,000 and 20,000magnifications, respectively, of a phosphor having zinc hydroxide formedin a suspension of the phosphor attached on its surface. FIGS. 4(a), (b)and (c) are diagrammatical views of a phosphor 1 corresponding to thephosphor of FIG. 1 and having zinc oxide 2 attached on its surface, aphosphor 1 corresponding to the phosphor of FIG. 2 and having zinc oxide2' deposited on its surface, and a phosphor 1 corresponding to thephosphor of FIG. 3 and having zinc hydroxide 3 attached on its surface.These phosphors were taken out from the respective suspensions byfiltration, followed by dehydration and drying.

It is evident from the above Figures that the phosphor of the presentinvention has zinc oxide consistently firmly attached thereto ascompared with the phosphor having zinc oxide particles directlydeposited. Further, zinc oxide attached to the phosphor of the presentinvention is different in the shape of attachment from the conventionalzinc hydroxide and is attached to the phosphor surface in a relativelyuniformly distributed state.

The phosphor of the present invention may be the one having zinc oxideformed as described above and an assistant for stabilizing theattachment of zinc oxide colloidal substance attached on its surface.The assistant for stabilizing the attachment may be, for example, zinchydroxide, aluminum hydroxide, alumina sol, zinc phosphate, magnesiumphosphate, aluminum phosphate, barium phosphate, calcium phosphate, zincpyrophosphate, calcium pyrophosphate, magnesium pyrophosphate, aluminumpyrophosphate, colloidal silica, ionic silica (water glass) or powderysilica. Such an assistant for stabilizing the attachment may beincorporated to a phosphor suspension prior to incorporation of zincions to the suspension so that it may preliminarily deposited on thephosphor surface, or it may be deposited into the phosphor surface towhich zinc oxide has already been attached.

A method for treating the surface of a phosphor to prepare the phosphorof the present invention comprises maintaining a phosphor suspensioncontaining a phosphor and zinc ions at a predetermined temperature, andadding an alkaline solution to the suspension to adjust the pH to apredetermined level, so that zinc oxide colloidal substance is formed inthe suspension, and zinc oxide is attached to the surface of thephosphor. Formation of the zinc oxide colloidal substance depends on thetemperature and the pH of the suspension. Therefore, maintaining thesuspension at a predetermined temperature and adjusting the pH to apredetermined level, means that the temperature and the pH of thesuspension are maintained under the conditions where zinc oxidecolloidal substance is formed. In general, the higher the temperatureand the higher the pH, the more readily zinc oxide tends to form. Thespecific temperature and pH ranges vary depending upon the type of thezinc ions solution to be added to the phosphor suspension (i.e. thecondition under which zinc oxide starts to form, varies depending uponthe type of the anion in the solution i.e. whether it is NO₃, SO₄ ²⁻,Cl⁻, CH₃ COO⁻⁻). However, the temperature and the pH at which zinc oxidemay be formed by a zinc ion solution by the method of the presentinvention, are within the range shown by the hatched line portion inFIG. 5. This range is a higher temperature and higher pH range ascompared with the above-mentioned case for forming zinc hydroxide. Whena zinc nitrate solution is used as the zinc ion solution, the range inwhich zinc oxide is formed will be close to the oblique boundarystraight line in FIG. 5. Whereas, when a zinc sulfate solution is usedas the zinc ion solution, the range in which zinc oxide can be formedwill be only at a relatively high temperature and high pH portion withinthe hatched line range in FIG. 5. With each zinc ion solution, zinchydroxide will be formed at a low temperature and low pH rangecontinuous from the zinc oxide forming range. Namely, in the case ofusing a zinc nitrate solution, a basic zinc nitrate [Zn(NO₃)_(x)·(OH)_(y) ] will be formed, and in the case of using a zinc sulfatesolution, a basic zinc sulfate [Zn(SO₄)_(x) ·OH_(y) ] will be formed. Itis possible to form zinc oxide by shifting the temperature and the pH ofthe suspension to the zinc oxide forming range even after formation ofsuch zinc hydroxide (hereinafter referred to as Zn(OH)²). In a highlyalkaline range of pH 13 or higher, zinc hydroxide and zinc oxide will bedissolved again to form ZnO₂ ²⁻ ions.

Now, Examples of the present invention will be described.

The phosphor of the present invention can be prepared by a method asdescribed below.

Firstly, as an example, a phosphor is put in deionized water of about70° C. and thoroughly suspended. Then, to this suspension, a solution ofe.g. zinc nitrate [Zn(NO₃)₂ ], zinc acetate [Zn(CH₃ COO)₂ ], zincchloride (ZnCl₂) or zinc sulfate (ZnSO₄) containing zinc ions, is addedin a suitable amount, and the mixture is again suspended. Among theabove zinc ion solutions, a solution of zinc nitrate or zinc acetate ispreferred since it is thereby possible to form zinc oxide at arelatively low pH and low temperature range as described hereinafter.Then, an alkaline solution is added to this suspension. At that time,the temperature of the suspension is controlled to be a temperaturewithin the zinc oxide forming range depending upon the pH value adjustedby an alkaline solution. For the alkaline solution, sodium hydroxide(NaOH), potassium hydroxide (KOH) or ammonium hydroxide (NH₄ OH) may beemployed. When the pH of the phosphor suspension exceeds a certain levelby the addition of this alkaline solution, zinc oxide ZnO will beprecipitated. Precipitated fine particles of ZnO will be attached on thesurface of the phosphor. Then, the suspension is left to stand to letthe phosphor having fine particles of ZnO attached thereto sediment.Then, the supernatant is removed by decantation. The remaining ions areremoved by repeating washing with deionized water a few times, followedby dehydration and drying at a temperature of from 100° to 150° C. Afterdrying, a bulky phosphor obtained is disintegrated by passing through ascreen to obtain the desired phosphor.

The substance attached on the phosphor by the above described operation,was forcibly peeled off by supersonic treatment, separated from thephosphor by sedimentation and then analyzed by X-rays, whereby it wasconfirmed that from the initial state of attachment to the phosphorsurface, the attached substance was ZnO irrespective of the temperaturefor drying treatment. The phosphor for color television picture tubes tobe used in the present invention, includes all kinds of phosphors usefulas the blue, green or red emitting component phosphor for a colortelevision picture tube phosphor screen. This phosphor for colortelevision picture tubes, may be a single phosphor, or a phosphormixture such as a green emitting component phosphor composed of amixture of a copper and aluminum-activated zinc sulfide phosphor(ZnS:Cu,Al) and a gold and aluminum activated zinc sulfide phosphor(ZnS:Au,Al), which has been practically used recently. Further, recentlya so-called pigment-attached phosphor which is a phosphor having itssurface coated with pigment particles is used for a phosphor screen of ahigh contrast color television picture tube. The phosphor for colortelevision picture tubes to be used in the present invention may also besuch a pigment-attached phosphor.

Particularly preferred phosphors for color television from a practicalviewpoint are as follows: as blue emitting phosphors, there can bementioned a silver activated zinc sulfide phosphor (ZnS:Ag), a silverand aluminum activated zinc sulfide phosphor (ZnS:Ag,Al), a cobaltaluminate blue pigment particle attached ZnS:Ag phosphor, a cobaltaluminate blue pigment particle attached ZnS:Ag,Al phosphor, etc.; asgreen emitting phosphors, there can be mentioned mixed phosphor of aZnS:Cu,Al phosphor and a ZnS:Au,Al phosphor, a ZnS:Cu,Al phosphor, agold, copper and aluminum activated zinc sulfide phosphor(ZnS:Au,Cu,Al), a copper and aluminum activated zinc-cadmium sulfidephosphor [(Zn,Cd)S:Cu,Al)], etc.: and as red emitting phosphors, therecan be mentioned a europium activated yttrium oxisulfide phosphor (Y₂ O₂S:Eu), a europium activated yttrium oxide phosphor (Y₂ O₂ :Eu), a redoxide red pigment particle attached Y₂ O₂ S:Eu phosphor, a red oxide redpigment particle attached Y₂ O₃ :Eu phosphor, a cadmium sulfoselenidered pigment particle attached Y₂ O₂ S:Eu phosphor, a cadmiumsulfoselenide red pigment particle attached Y₂ O₃ Eu phosphor, etc.

Now, the color mixing preventing effects of the phosphor of the presentinvention will be described.

Color mixing of light emitting phosphor components occurs when a secondlight emitting phosphor component or a third light emitting phosphorcomponent is slurry-coated, exposed, and developed to form dots orstripes of the phosphor component, the phosphor component attaches todots or stripes of another light emitting phosphor component alreadyformed and remains there. Such color mixing is called "crosscontamination".

The solid line curves in FIGS. 6 and 7 show the relations between theamount of attached ZnO of a ZnS:Ag phosphor and a Y₂ O₂ S:Eu phosphorhaving ZnO attached thereto, and the cross contamination of thesephosphors with other light emitting phosphor components. Here, the crosscontamination means cross contamination to a light emitting phosphorcomponent already coated. FIG. 6 is a graph showing the relation betweena post-coated ZnS;Ag phosphor having the amount of the attached ZnOvaried and the cross contamination of that phosphor with pre-coatedZnS:Cu,Al green emitting phosphor component. FIG. 7 is a graph showingthe relation between a post-coated Y₂ O₂ S:Eu red emitting phosphorcomponent having the amount of the attached ZnO varied and the crosscontamination of that phosphor with a pre-coated ZnS:Ag phosphor. InFIGS. 6 and 7, the cross contamination (ordinate) is represented by thevalues of blue output/green output and red output/blue output,respectively. These values were measured as follows.

In the case of FIG. 6, a slurry of a ZnO-attached ZnS-Ag phosphor samplewas coated on a face plate having stripes of a ZnS:Cu,Al green emittingphosphor component (untreated with ZnO) formed beforehand, then driedand developed without performing light exposure. Thereafter (as a matterof course, there was no formation of stripes of a ZnS:Ag phosphor), thestripes of the ZnS:Cu,Al phosphor were excited by ultraviolet ray of3650 A, the emission thus emitted was divided into two portions by ahalf mirror, the two lights thus divided were passed through green andblue Wrattern filters respectively, and the output of each light wasmeasured by means of a photomultiplier, whereby the value ofblue-output/green-output was determined. The value ofblue-output/green-output was determined for each phosphor sample and thevalues were plotted against the amounts of ZnO (parts by weight per 100parts by weight of a ZnS:Ag phosphor) of the phosphor samples, takingthe value of the phosphor having no ZnO as 1.

In FIG. 6, as Comparative Examples, the alternate long and short dashline shows a case wherein a ZnS:Ag phosphor having ZnO deposited on thephosphor surface by incorporating ZnO particles to the ZnS:Agsuspension, was employed, and the dashed line indicates a case wherein aZnS:Ag phosphor having Zn(OH)₂ attached thereto instead of ZnO, wasemployed. With respect to Zn(OH)₂, the weight of Zn(OH)2 is plotted ascalculated as the weight ZnO. Hereinafter, fixing of ZnO or Zn(OH)₂formed in the suspension to the phosphor surface will be referred to as"attaching", and fixing of ZnO incorporated to the suspension on thephosphor surface will be referred to as "depositing".

In the case of FIG. 7, firstly a ZnS:Ag phosphor sample was slurrycoated, exposed, and developed to form stripes. Then, on the face platehaving the stripes of this sample formed, a slurry of a ZnO-attached Y₂O₂ S:Eu red emitting phosphor component was coated, dried and thendeveloped with warm water without performing light exposure. Thereafter(as a matter of course, there was no formation of stripes of the Y₂ O₂S:Eu phosphor), the stripes of the phosphor sample were excited byultraviolet ray of 3650 A, the emission thus emitted was divided intotwo portions by a half mirror, the two lights thus divided were passedthrough blue and red Wrattern filters, respectively, and the output ofeach light was measured by means of a photomultiplier, whereby the valueof red-output/green-output was determined. The value ofred-output/green-output was determined on each phosphor sample and thevalues thus obtained were plotted against the amounts of ZnO (parts byweight per 100 parts by weight of the ZnS: Ag phosphor) of the phosphorsamples, taking the value of the phosphor having no ZnO as 1.

Also in FIG. 7, as Comparative Examples, the alternate long and shortdash line shows a case wherein a Y₂ O₂ S:Eu phosphor having ZnOdeposited on the phosphor surface by incorporating ZnO particles to theZnS:Ag suspension, was employed, and the dashed line shows a casewherein a Y₂ O₂ S:Eu phosphor having Zn(OH)₂ attached instead of ZnO,was employed. With respect to Zn(OH)₂, the weight of Zn(OH)₂ was plottedas calculated as the weight of ZnO. In FIGS. 6 and 7, the larger thevalues of blue output/green-output and red-output/blue-output, thegreater the cross contamination of the ZnS:Ag phosphor to the pre coatedZnS:Cu,Al phosphor and the cross contamination of the post-coated Y₂ O₂S:Eu phosphor to the ZnS:Ag phosphor.

As is evident from FIGS. 6 and 7, with the ZnO-attached phosphors of thepresent invention, the cross contamination can be reduced as comparedwith the ZnO-deposited phosphors (alternate long and short dash line)and the Zn(OH)₂ attached phosphors (dashed line). The effects forreducing the cross contamination are particularly remarkable in theregion where the attached amount is greater than the Zn(OH)₂ method.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted by such specific Examples.

EXAMPLE 1

In 3 l of deionized water of 70° C., 1,000 g of a ZnS:Ag blue emittingphosphor was put and thoroughly suspended. Then, 60 ml of a 10% zincnitrate [Zn(NO₃)₂.6H₂ O] solution was added thereto, and the mixture wasagain thoroughly stirred and suspended. Then, while maintaining thetemperature of the suspension at 65° C., a 2% NaOH solution wasgradually added under stirring to adjust the pH to 8.5. Thereafter,stirring was continued for 30 minutes, and then the mixture was left tostand still for 10 minutes for sedimentation of the phosphor. Then, thesupernatant was removed by decantation, and the mixture was washed oncewith deionized water. Then, the phosphor was collected by filtration,dehydrated, dried at 120° C. for 15 hours and then sieved with a 300mesh sieve.

The substance attached on the surface of the phosphor thus obtained waspeeled off by supersonic treatment, collected and analyzed by X-rays,whereby it was ascertained to be ZnO. The amount of attached ZnO was0.15 part by weight per 100 parts by weight of the phosphor.

Then, a phosphor slurry was prepared in a usual method by using thephosphor obtained as described above and an ordinary aqueous ammoniumbichromate-containing polyvinyl alcohol aqueous solution. A screeningtest was performed using the phosphor slurry thus obtained, whereby thelight sensitivity of the phosphor layer, the adhesiveness to the paneland the color mixing (contamination represented by theblue-output/green-output) were examined. The results are shown in Table1.

As Comparative Examples, similar tests were performed with respect to aZnS:Ag phosphor having 0.15% by weight of Zn(OH)₂ attached thereto and aZnS:Ag phosphor having 0.15% by weight of ZnO deposited thereon.

The coating test in this example was conducted in such a manner that acircular filter having the angle and the ultraviolet transmittancevaried, was mounted in front of a shadow mask forming stripe pictureelements, and a predetermined dose of ultraviolet rays were uniformlyirradiated for exposure, followed by development with water, whereuponthe state of adhesion of the picture elements (stripes) on the plate wasinspected. The adhesion angle in Table 1 is meant for the angle of afan-shaped portion from the position on the plate of the circular filterwhere it overlaps with the position where the ultraviolet transmittanceis the highest (as a matter of course, the phosphor screen at thisposition is attached in the most stable state) to the position on theplate where the picture elements start to fall as the ultraviolettransmittance decreases. The larger the angle, the better thesensitivity and the adhesion to the plate. Further, the maximum stripewidth is the width of the stripe at the position where the ultravioletexposure was highest. The larger this width, the higher the sensitivity.The minimum stripe width means the minimum stripe width of the stripeformed in the region where the ultraviolet exposure was reduced by theabove circular filter. The smaller this width, the better theadhesiveness. Further, as mentioned above, the cross contaminationindicates the degree of color mixing as determined in such a manner thatafter forming stripes with a green emitting phosphor, a ZnS:Ag blueemitting phosphor is coated and developed without exposure, and then,the ratio in the emission of the green fluorescence to the bluefluorescence under ultraviolet radiation is obtained.

                                      TABLE 1                                     __________________________________________________________________________    Screening properties                                                          Phosphor                                                                            Amount of   Stripe                                                                              Stripe                                                (ZnS:Ag                                                                             ZnO attached                                                                         Adhesion                                                                           width width Cross contamination                             phosphor)                                                                           (or deposited)                                                                       angle                                                                              (maximum)                                                                           (minimum)                                                                           Red output/blue output                          __________________________________________________________________________    ZnO   0.15 wt %                                                                            250°                                                                        220 μm                                                                           130 μm                                                                           0.74                                            attached                                                                      Zn(OH).sub.2                                                                        0.15 wt %                                                                            200°                                                                        200 μm                                                                           150 μm                                                                           1                                               attached                                                                      ZnO   0.15 wt %                                                                            210°                                                                        210 μm                                                                           135 μm                                                                           1.28                                            deposited                                                                     __________________________________________________________________________

It is evident from the above results, the phosphor of the presentinvention has higher sensitivity and adhesiveness than the conventionalphosphor having Zn(OH)₂ attached thereto and is less likely to lead tocolor mixing. It has also been confirmed that in order to obtain such ahigh quality phosphor, it is not sufficient to merely deposit ZnO on thephosphor surface, but it is necessary to attach ZnO formed in a solutionto the phosphor surface.

EXAMPLE 2

In 3 l of deionized water of 70° C., 1,000 g of Y₂ O₂ S:Eu red emittingphosphor was put and thoroughly suspended. Then, 120 ml of a 10%-zincnitrate [Zn(NO₃)₂.6H₂ O] solution was added thereto, and the mixture wasagain thoroughly stirred and suspended. Then, while maintaining thetemperature of the suspension at 65° C., a 2.8% NH₄ OH solution wasgradually added under stirring to adjust the pH to 8.5. Thereafter,stirring was continued for 30 minutes, and then the mixture was left tostand still for sedimentation of the phosphor. Then, the supernatant wasremoved by decantation, and the mixture was washed once with deionizedwater. Then, the phosphor was collected by filtration, dehydrated, driedat 120° C. for 15 hours and sieved with a 300 mesh sieve.

The substance attached on the surface of the phosphor thus obtained waspeeled off by supersonic treatment, collected and analyzed by X-rays,whereby it was ascertained to be ZnO. The amount of attached ZnO was 0.3parts by weight relative to 100 parts by weight of the phosphor.

Then, a phosphor slurry was prepared by a usual method by using thephosphor obtained as described above and an ordinary ammoniumbichromate-containing polyvinyl alcohol aqueous solution. A screeningtest was conducted using the phosphor slurry thus obtained, whereby thesensitivity, the adhesion to the panel, the color mixing (crosscontamination represented by the red output/blue output) and the powderproperty (dispersibility) were examined. The results are shown in Table2. As Comparative Examples,-in the same manner as in Example 1, similartests were conducted also with respect to a Y₂ O₂ S:Eu red emittingphosphor having 0.3% by weight of Zn(OH)₂ attached thereto and a Y₂ O₂S:Eu red emitting phosphor having 0.3% by weight of ZnO depositedthereon. The sedimentation volume in water in the Table was determinedin such a manner that 5 g of the above-mentioned phosphor sample was putin 30 g of an aqueous solution and sedimented after shaking asedimentation tube for one hour, whereupon the volume was read, and thesedimentation volume in water was represented by the volume per g. Thelarger the value of the sedimentation volume, the poorer thedispersibility. The water wettability was determined in such a mannerthat water was added to the phosphor while phosphor was vibrated, andthe amount of water required to completely wet the phosphor was taken asthe water wettability. The smaller the required amount of water, themore easily the slurry can be prepared. With respect to the hardness ofthe dried cake, the softer the cake, the better the dispersibility ofthe phosphor.

                                      TABLE 2                                     __________________________________________________________________________                 Screening properties         Powder properties                   Phosphor                                                                            Amount of    Stripe                                                                              Stripe                                                                              Cross Contamination                                                                      Sedimentation  Hardness of          (Y.sub.2 O.sub.2 S:Eu                                                               ZnO attached                                                                         Adhesion                                                                            width width Red output/                                                                              volume in water                                                                        Water dried                phosphor)                                                                           (or deposited)                                                                       angle (maximum)                                                                           (minimum)                                                                           blue output                                                                              (cc/g)   wettability                                                                         cake                 __________________________________________________________________________    ZnO   0.3 wt %                                                                             260°                                                                         220 μm                                                                           130 μm                                                                           0.68       0.48     14.6 wt                                                                             Soft                 attached                                  (Supernatant;                                                                 transparent)                        Zn(OH).sub.2                                                                        0.3 wt %                                                                             200°                                                                         220 μm                                                                           155 μm                                                                           1          0.56     16.0 wt                                                                             Hard                 attached                                  (Supernatant;                                                                 transparent)                        ZnO   0.3 wt %                                                                             200°                                                                         200 μm                                                                           140 μm                                                                           1.35       0.52     16.4 wt                                                                             Soft                 deposited                                 (Supernatant;                                                                 turbid)                             __________________________________________________________________________

As is evident from the above results, it has been confirmed that thephosphor of this Example is excellent in each of the light sensitivity,the adhesiveness and the dispersibility and is less susceptible to colormixing. Especially with respect to the effect for preventing colormixing, in a case where Zn(OH)₂ is attached, the effect tends todeteriorate when the amount of attachment is as much as 0.3% by weight,whereas with the phosphor of the present Example, adequate effects forpreventing color mixing have been accomplished even when the amount ofattachment of ZnO is relatively large.

EXAMPLE 3

In 3 l of deionized water at 70° %C, 1,000 g of a ZnS:Cu,Au,Al greenemitting phosphor was put and thoroughly suspended. Then, colloidalsilica was added thereto in an amount of 0.5 part by weight relative to100 parts by weight of the phosphor, and the mixture was thoroughlystirred and dispersed. Then, 60 ml of a 10% zinc nitrate [Zn(NO₃)₂.6H₂O] was added, and the mixture was thoroughly stirred and dispersed.Then, while maintaining the temperature of the suspension at 65° C., a2% NaOH solution was gradually added under stirring to adjust the pH to8.5 g. Then, the stirring was continued for 30 minutes, and then themixture was let to stand still for 10 minutes to let the phosphorsediment. Then, the supernatant was removed by decantation, and themixture was washed once with deionized water. Then, the phosphor wascollected by filtration, dehydrated, dried at 120° C. for 15 hours andsieved with a 300 mesh sieve.

The substance attached to the surface of the phosphor thus obtained, waspeeled off by supersonic treatment, collected and analyzed by X-rays,whereupon the substance was ascertained to be composed of SiO₂ and ZnO.Further, the amount of attached SiO₂ was 0.4 part by weight relative to100 parts by weight of the phosphor, and the amount of attached ZnO was0.18% by weight. A phosphor slurry was prepared by a usual method byusing the phosphor obtained as described above and an ordinary ammoniumbichromate-containing polyvinyl alcohol aqueous solution. A coating testwas conducted using the phosphor slurry thus obtained, and the lightsensitivity, the adhesion to the panel, the water wettability and thefogging of the glass surface were examined. The results are shown inTable 1. As a Comparative Example, a similar test was conducted withrespect to a ZnS:Cu,Au,Al phosphor having 0.18% by weight of Zn(OH)₂attached instead of ZnO.

The fogging of the glass surface is meant for color mixing caused bydots or stripes of the above-mentioned green phosphor remaining at theposition of dots or stripes of a light emitting phosphor componentformed subsequently. In this test, as described above (see Example 1), apart of a face plate having stripes formed thereon, is enlarged (100times by a magnifying projection microscope and projected on a screen,whereupon the number of phosphor particles remaining in the non-exposedportion, i.e. in the space portion between stripes, in 20 mm×20 mm onthe screen (0.2 mm×0.2 mm on the face plate).

                                      TABLE 3                                     __________________________________________________________________________                       Screening properties      Powder                           Phosphor Amount                                                                             Amount    Stripe                                                                              Stripe         properties                       (ZnS:CU,AU,Al                                                                          of ZnO                                                                             of SiO.sub.2                                                                       Adhesion                                                                           width width Fogging of                                                                             Water                            phosphor)                                                                              attached                                                                           deposited                                                                          angle                                                                              (maximum)                                                                           (minimum)                                                                           glass surface                                                                          wettability                      __________________________________________________________________________    ZnO, SiO.sub.2                                                                         0.18 0.40 200°                                                                        230 μm                                                                           130 μm                                                                            5 particles/                                                                          15.0 wt %                        coated   wt % wt %                  0.2 mm × 0.2 mm                     Zn(OH).sub.2,                                                                          0.18 0.40 150°                                                                        200 μm                                                                           145 μm                                                                           15 particles/                                                                          17.5 wt %                        SiO.sub.2 coated                                                                       wt % wt %                  0.2 mm × 0.2 mm                     __________________________________________________________________________

As is evident from above results, with the phosphor of this Example,better results were obtained in the respective tested items as comparedwith the phosphor coated with Zn(OH)₂ and SiO₂.

As described in detail in the foregoing, according to the phosphor ofthe present invention and the method for treating the surface thereof,it is possible to accomplish the prevention of color mixing of thephosphor screen and the improvement of the adhesive strengthsimultaneously by attaching to the phosphor surface zinc oxide formed ina suspension of the phosphor. Further, the dispersibility of thephosphor can be maintained in good condition even when the attachedamount is increased.

We claim:
 1. A phosphor composition comprising a phosphor for a colorcathode ray tube and zinc oxide attached to the surface of the phosphor,prepared by a method which comprises forming an aqueous phosphorsuspension containing a phosphor and zinc ions, adding an alkalinesolution to the suspension to raise the pH of the suspension, andraising or maintaining the temperature of the solution at a level suchthat colloidal zinc oxide is formed in the suspension and attached ontothe surface of the phosphor; the pH and temperature during the formationand attachment of the colloidal zinc oxide to the phosphor being withinthe hatched line portion of FIG. 5 of the drawings.
 2. A phosphorcomposition as claimed in claim 1, wherein an assistant for stabilizingthe attachment of the colloidal zinc oxide to the surface of thephosphor is present in said aqueous phosphor suspension, said assistantbeing selected from the group consisting of zinc hydroxide, aluminumhydroxide, alumina sol, zinc phosphate, magnesium phosphate, aluminumphosphate, barium phosphate, calcium phosphate, zinc pyrophosphate,calcium pyrophosphate, magnesium pyrophosphate, aluminum pyrophosphate,colloidal silica, water glass, powdery silica, and mixtures thereof. 3.A phosphor composition according to claim 1, wherein the zinc ionsinitially present in the phosphor suspension are supplied by zincnitrate, alkaline solution is added to the suspension to adjust the pHto 8.5, the suspension is maintained at a temperature of 65° C., and thephosphor having colloidal zinc oxide attached to the its surface isrecovered and dried at 120° C.